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    Experience of working p8 Experience of working p8 Document Transcript

    • LONGWALL UNDER r maADVERSE STRATACONDITIONS Ku shEXPERIENCE OF WORKING PANEL NO.8 ure D.S r:MMD.SURESH KUMARAdditional Manger/Colliery ManagerThe Singareni Collieries Company Limited, Kothagudem, A.P. tho Au
    • INDEX1. Introduction r ma2. General Information of Longwall Panel No.83. Reducing the Face length Ku Supports in the Face Face Operation sh Face Swing4. Working the Face in 1 in 4 gradient ure Problems faced with Face Steepness Creep Control D.S5. Negotiating the dyke in the Coal Seam Face advance below Dyke r:M6. Water bearing strata and Goaf water from overlying Mine Dealing the Face Water Effect of Heavy inrush of water from the roof strata tho7. Geological Disturbances8. Working over Virgin Bottom Seam Au Figures to support the workings over virgin Bottom Seam9. Longwall Face Retreat below High roof Stone Roof Portion Supporting in the High Roof Method of Girder Support in the High roof Face Retreat below high roof Floor Heightening10. Conclusions11. References
    • 1. INTRODUCTION No.5 Incline, Kothagudem Area is one of the biggest underground mines of SCCL,Khammam district, Andhra Pradesh. There are three workable seams namely Queen seam (Topseam) King seam and Bottom seam. The mine was started in the year 1952.Extensive development and depillaring wasmade in King Seam and bottom seam. A vertical shaft of 212 m was sunk in the year 1971/1972and equipped with winding arrangement. Subsequently chair lift system was also introducedduring 1999, which reduced traveling length. r With bilateral agreement between India and China, two Longwall sets with 4X760 mate IFS Powered roof supports and 2 km long steel card belt supplied by M/s CME China and putthem into operation in the mine at Top seam. Out of that one unit has been transferred to GDK 9 KuInc RG II Area. Longwall panels have been prepared in Top seam having ‘F’ grade coal. The King shand bottom seams are containing high-grade coal D & C grade, which were worked extensivelyby conventional board and pillar mining upto 280 m depth. AM-50s are deployed for LW panels urepreparation in top seam. So far 11 LW panels have been worked in top seam in middle sectionThe 12th Panel (i.e. panel No 8) was completed recently. D.S Two sets of Longwall units with 2Km long steel cord belting for coal evacuationfrom underground to surface were introduced in the year 1995-96.LW unit-I had worked panels2, 4, 5A and 5D and unit II had worked panels 3, 5, 5C, 22, 1, 1A, 21 and 8.There by each unit r:Mhas produced 19, 71,874 Te and 27, 75,095 Te respectively. As per the feasibility report there were 26 panels (15.4 M te) proposed in top seam and tho14 panels (9.63Mte) in King seam .Out of 26 panels in Top seam, 11 have been completed. Thebalance 14 panels have been reduced to 7 due to the following reasons, Au 120-m face length as proposed in FR was re-drawn to 150 m. Change in fault position and its throw, as per latest geological Information. In the Virgin King seam 14 panels have been projected in ‘FR’.However 7 panels are available below the panels in Top seam. Finally, the following are LWpanels available for extraction in the Mine. Top seam Panels 7 9 10 11 12 13 14 Total Reserves 5.06 4.14 4.60 3.52 3.50 3.54 6.94 31.33 King seam Panels 8 9 10 11 12 13 14 1.6 M Te
    • 2. GENERAL INFORMATION OF LONGWALL PANEL No 82.1 Panel Details:1. Face length : 150m Center to Center2. Panel length : 420m3. Depth of the Top seam : 275m -305m4. Thickness of the Seam & Height of : 9.0m & 3.0m extraction5. Face Gradient : 1 in 6 and 1 in 4 r ma6. Immediate Roof & Floor : Shaley Coal7. Condition of underlying seam : King seam Virgin8. Condition of overlying Seam : No.1 Seam Goaf (i.e., 8A, 8B & 8C Panels) of 5B Ku Incline9. Number of supports in the face : 103 sh10. Coal Grade : Higher ‘G’, >2000Kcal Per Kg ure2.2 Powered Roof Support:1. Make : CME (China Mining Engineering), China2. Type : Chock Shield IFS (Immediate Forward Support) D.S3. Length : 3.87m (Main Canopy: 2.47m and Articulated Canopy : 1.40m)4. Width : 1.4m (1.50m Center to Center) r:M5. Weight : 20.5 tonnes6. Height : 2.20m (Fully closed piston) and 3.40m (Fully opened tho piston)7. Capacity : 4 Legs x 760 tonnes, each Leg 190 tonnes Au8. Pull Force : 63 tonnes9. Push Force : 30 tonnes10. Support Density / Resistance : 110 tonnes per square metre (Chock), 103 t/Sq.m. (Overall face)11. Setting Pressure : 26 Mega Pascals (MPa) or 260 Bar (68% of the Yield Pressure) 29 MPa or 290 Bar (75% of Yield Pressure) during weighting periods12. Yield Pressure : 38.70MPa or 387 Bar13. Leg Type : Single Telescopic with 1.20m piston length14. Safety Feature : Yield or Bleed valve (Rate:120 liters per minute)
    • 15. Leg make : CME,China and JBE (Jaya Bharat Engg Ltd.), India2.3 Strata The different beds of the strata lying over the working section of the longwall panel are shown below in Fig.1. The thickness of the individual beds of Grey sandstone (Grey Sst) is also shown in Fig.1. The thickness of the Shaley coal, i.e., immediate roof is 3.0m, and that of the clay band is 1.44m. r maBorehole section Ku1. Local fall is composed of fall of 3.0m shaley coal roof and 6.35m Grey sand stone bed. Fall of shaly coal occurred at 11.0m and shaly coal plus 6.35m sandstone bed occurred at 21.80m. sh2. Main fall occurred at an average progress of 80.30m with an area of exposure of 12840 square meters. ure3. Weighting zone during main fall : C38 to 1034. During main fall beds upto a strong bed (Moderately Cavable) of 14.68m have been D.S collapsed.5. The caving height in this panel is 24m (i.e., shaley coal 3.0m + Grey Sst bed 6.35m + Grey Sst bed 14.68m as shown in fig.1.). r:M6. Periodical caving of the main roof occurred at an interval of 15 to 25m. tho3. Reducing the face length As per ‘FR’, there were 11 Longwall panels have been extracted in the AuTop Seam. The Virgin dip side property kept untouched due to Neighbouring mine -5B. Henceit was planned to develop panels in the North side property behind the existing Longwall Panelgoaves. The length of the Panels in the Top seam mainly restricted by a 6.5m throw fault(No….) which is traversing from North West to South east. To reach the Virgin patch on Northside, Trunk roadways were planned to be developed by negotiating this fault. Therefore, adecision was taken to drive the Top gate and bottom gate upto fault No…., and to connect theface dip at 58m bottom gate advanced than Top gate to prove the fault and to extract theadditional property in the triangle portion.
    • The 58m staggering of gate roadways increased the face length by 166m.Though the difficulties added by the additional length 16m in the Face dip is well known, it wascarefully planned to swing the face to reduce it to 150m based on the experience of Panels No.5Cin the same Mine.3.1 Supports in the Face Normally, 102 PRS are installed in any of the Panels worked here with150m Face length. Whereas in Panel No.8 one additional support was added to take care of Gateroadway support due to anticipated creep problem because was the first Panel to be worked with rthe Face gradient of 1 in 4.5. But the extra length (16m) was planned to be supported with cogs maand OC Props. Keeping few more no of Power Supports was not feasible due to its removal andevacuation when face length reduced. Ku During installation of Power Supports, around 10 spaces of 1.5m lengthwere created by missing Power Supports then and there from Midface to Top gate side. But the shAFC Pans were continuously arranged, so that, the extra length was covered by introducingadditional Pans. The spaces in between the Power Supports were supported by bolting, meshing, ureone cog and OC Props. D.S3.2 Face Operation The Shearer could be traversed from bottom gate to Top gate and cut the r:Mentire face due to extra Pans added in the AFC. The Power Supports used to be advanced dip &Rise side of “Spaces” supported by Cogs and OC props which were used to shifted by one web tholength by stopping the Shearer. AFC Pan in that place was advanced with the help of adjacentPower Support. Problem of difficulties of AFC pushing occurred rarely when short length OC AuProps were utilized to push the AFC.3.3 Face Swing To reduce the face length gradually, the Face was swing by cutting theface at the ratio of 1:3 between bottom gate and Top gate respectively. On reduction in the facelength by every 1.5m / .3.0m, One or two pan sets were to be removed at top gate i.e.,immediate line pans below Drive unit and the gradient Pans. Drive unit were pushed andmeshed to the line pans at dip side. Thus two Power Supports at top gate became idle, withoutPans. The ‘Spaces’ left out in the Supports were covered with Supports by detaching all theClevis of Power Supports between nearest “Space” and top gate and by pushing them towards
    • dip. In this way the Face length was reduced gradually, Additional Pans were removed and the‘gaps’ in the Power Supports have been covered Stage wise.4.0 WORKING THE FACE AT 1 IN 4 GRADIENT Unlike in the other faces, General Seam steepness increased from 1 in 10 to 1 in 6in this Panel. Moreover, there was gradient change within the face of 150m that the coal seamfrom top gate to mid face formed as steep as 1 in 4 and 1 in 6 from mid face to bottom gate.Smooth face needed to be worked as per seam formation with two different gradients along theface. r4.1 Problems faced with Face Steepness ma4.1.1 Face alignment was quite problematic, as the total face length couldn’t be seen with eyes from one gate to other due to two different gradients in the face Ku4.1.2 The Power Supports in the steep portion of the face used to tilt towards bottom gate and hardly have they been maintained normal to the seam. Thus, in the both the ways the sh “support Resistance” was reduced due to “Upward support Résistance” was not being totally utilized to thrust the Roof vertically. ure4.1.3 Shearer traction shoe, Sprocket assembly and guide shoe started breaking frequently. High shearing load and load on hydraulic main pump, auxiliary pumps in the pump box D.S were experienced with the Chinese Shearer DERD-2 x 300 KW. Completion of P-8, has consumed quit number of Pumps, Sprocket assemblies and traction shoes. r:M4.1.4. Drop in Pressure in the Hydraulic Circuit at the Top gate was 4 -6 Mpa resulting in Sluggish hydraulic operation in the Power Supports. Two numbers of 200 GPM Chainco pumps were operated simultaneously for some times, to componsate the pressure drop. tho4.2 Creep Control Au4.2.1 No special technique was adopted for Creep control in the face. The bottom gate was kept 25m advanced by top gate that came to 9-10 degrees oblique face which was double the amount, in terms staggering length and creep angle when compared to other faces of flat gradient.4.2.2 While advancing the bottom gate, the Power Supports were always advanced with an anchoring prop pushing towards Rise side, to avoid tilting and sliding of Gate end Supports. Generally the AFC bottom drive pushed and kept anchored first then the other pans advanced from dip to Rise.
    • 4.2.3 Much comfort was found in Bidirectional Cutting rather than Unidirectional Cutting that advancing Power Supports and AFC simultaneously had less slope for turning or sliding of Supports towards dip direction.4.2.4 In some occasions AFC top gate drive was pulled towards Rise and anchored with a conveyor advancing ram which has also given better result.5.0. NEGOTIATING A “DYKE IN THE COAL SEAMAt …….. m of face retreat, the Sand stone roof lowered down for 3m from C-75 to C-85 for the rlengths of 15m. The top 3m, Shale and Shally Coal portion of the Seam got missing but the mamiddle 3m portion which is working section was intact. Ku sh ure D.S r:M tho Hence, it was planned to work the face having Smooth Aufloor horizon and with the Stone roof as contact over the Supports5.1 Face advance below Dyke5.1.1 The uneven shape of the Sand stone roof in this portion was cut with Shearer to maintain uniform roof and floor horizons.5.1.2 Lamination of coal seam with the Sand stone portion at the either edges of the dyke was found very weak and sandy in nature, which was incapable of withstanding to the abutment load particularly during face-weighting period. The either sides of the dyke used to collapse readily resulting in “Face Cavity”.5.1.3 This was overcome by introducing wire mesh in that portion with fore-pole bolts, for the length of …… m along the strike direction of the face.
    • 6.0. WATER BEARING STRATA AND GOAF WATER FROM OVERLYING MINE A Seasonal nalla known as “Tellavagu” crossing the Mine property at dip side. Due tothat, the strata were reasonably charged with water below 110L. Moreover the Panel No.8 wasexactly located below goaves of Panel No.8A, 8B, 8C of Neighbouring Mine-5B. The averagepartition between both the seams is 190m. The make of water during development of Panel itselfwas 400 GPM. It was increased gradually to 600 GPM before main fall, 900 GPM after mainfall and sustained at 1000 GPM for a long period after 3 - 4 Periodic falls. The maximum outflow of water was 1300 GPM during a Periodic fall. The unexpected water outflow from the Panel No.8, redefined the entire Pumping r maarrangement of the Mine6.1 Dealing the Face water Ku6.1.1 A deep nallah was cut manually along the bottom gateway as practiced in the earlier Panels, Spending more than 1000 Manshifts during Panel – Preparation, which was sh effectively utilized to deal with the inrush of goaf water by cleaning it regularly.6.1.2 Water- Pockets were prepared with the dimension of 4m length and 2.5m depth by ure blasting the dip side barrier at three places along the length of the gate roadways. Setting Chambers have been constructed in the Pockets which were cleaned regularly to send the D.S clean water without slush to the pumping station.6.1.3 The dip most workings were not prepared to deal with extra water influx from Longwall r:M Panel. Standage capacity was less as there were limited galleries in the dip side. Hence, a small pumping station was set at the entrance of the gate roadways and the water from the Panel used to be pumped out with 1 – 40 HP (250 GPM) high head pump and 1- 75 tho HP (500 GPM) low head pump. Spare pumps were kept readily available at the site itself. Au6.1.4 The total Mine water make up of 1800 GPM without Panel No.8 was added by another 1000 GPM from Panel No.8 which warranted the introduction of 2 No’s of 350 HP Pumps apart from existing 5 No’s of 240 HP Pumps.
    • 1 IN ar N um hK res Face Start Line Face Retreat LONGWALL PANEL # 8 420m X 150m Su D. A A r:M thoPlan of Longwall panel No 8 in Top seam of No.5 Incline over lined by Goaved out workings of No.1 Seam of 5B Incline Au
    • 6.2 Effect of Heavy Inrush of Water from the Roof- Strata6.2.1 There was water always dripping from Roof-Strata, over the Canopies in the face particularly midface to bottom gate.6.2.2 The continuous water percolation from the strata considerably weakened the immediate coal and Shale Portion which used to readily crush over the canopies.6.2.3 The Shale part of the overlying roof, on mixing with strata water, due to abutment load during face weighting period, became a loose-mass and was collapsing readily for face cavities.6.2.4 50 % of Periodic Weightings resulting in face Cavities due to few more reasons in r ma conjunction with ‘excess strata water’.6.2.5 The running mass of Shale and Clay mixed with water, on falling down onto AFC, stalled the 2x200 KW AFC on many occasions. The time taken to restart the AFC after manual Ku cleaning the coal, still aggravated the Cavity situation. sh7.0 GEO-LOGICAL DISTURBANCES ure This was the first Longwall Panel in the Mine which experienced severe geological that ……….. No. of minor faults with 0.1m to 1.0m throw, …… No. of Slips and uncountable D.S number of joint-planes along Face dip parallel to the face. Gate roadways were found with so many undulations due to number of up throw and down throw faults. Gallery heights reduced at some places in bottom gate which were heightened by dropping the roof layers. r:M Two major roof collapses occurred at bottom gate during such heightening due to existence of combination of slips and minor faults. tho8.0 WORKING OVER VIRGIN BOTTOM SEAM Au
    • 8.1.1 FIGURES TO SUPPORT THE WORKINGS OVER VIRGIN BOTTOM SEAM F ro n t R ea r 2 5 Leg Pressure (MPa) 2 0 1 5 1 0 2 5 5 0 7 5 1 0 0 1 2 5 1 5 0 r F a c e P r o g r e ss (m ) ma Fig. Pressure distribution between Front and Rear legs Periodic weighing - 4 Periodic weighing - 3 Ku Periodic Weighting -1 Periodic weighting - 2 Main weighting 75 70 sh Mean Load Density ( t / sq.m) 65 ure 60 55 D.S 50 45 40 r:M 25 50 75 100 125 150 F a c e P r o g r e ss (m ) Fig. Mean Load density curve in Longwall panel # 8 tho 40 CPR 75 30 Au 20LgPes r ( P ) e r sue M a 40 CPR 65 30 20 40 CPR 50 30 20 40 CPR 30 30 20 20 40 60 80 100 120 Face Progress (m) Fig. Continuous Pressure survey along the length of the panel # 8.
    • 8 0 T o p Z o n e 7 0 MLD ( t / sq.m ) 6 0 5 0 4 0 3 0 8 0 M id Z o n e 7 0MLD ( t / sq.m ) r 6 0 ma 5 0 4 0 Ku 3 0 8 0 B o tto m Z o n e 7 0 MLD ( T / sq.m ) sh 6 0 5 0 ure 4 0 3 0 2 5 5 0 7 5 1 0 0 1 2 5 1 5 0 D.S F a c e P ro g re s s (m ) r:M Fig. Zone-wise Meal Load Density Curve in Longwall Panel tho Au
    • 9.0 LONGWALL RETREAT BELOW HIGH ROOF During formation of the Panel No.8, the tailgate 111L was driven with different roof sections, due to series of up throw and down throw faults, particularly the portion of tailgate (i.e.305M to 400M – 95M length) was developed in top section having sand stone roof. 9.1 Stone roof Portion: Length : 305m to 400m (95m) Avg. width of the gallery : 4.0m r ma 5.5m (370m – 385m) at roof horizon Height : 3.5m to 6.5m Sides : Week Ku Condition of Sandstone : sounds good, Bolted with mesh sh 9.2 Supporting in the High roof 1. Additional bolting is being done with special capsules to have anchorage in watery ure conditions. Around 150No’s new bolts are required to be foxed with 1.0m grid pattern. 2. Two stage girders fixed at 2.0m interval with 3/6 laggings D.S 3. Timber spread over the 3/6 lagging with wire mesh. 4. Wooden cogs with special timber constructed at 2.0m apart from I – Stage girders to II – r:M Stage girders and jammed to stone roof. 5. Lower stage girders were fixed in such a way to maintain optimum height of tailgate chocks i.e. 30Cms above the long wall face roof. tho 6. Some additional bolts with mesh fixed to the sides between I – Stage girders and stone roof and this is being continued. Au 7. The lower stage girders are reinforced by tying each other with 3/6 girders and bolting. 8. All girders notches were grouted with cement. 9. 40T OC Props are fixed below the girders as per SSR. 10. 40 Te OC Props are being erected in the abutment zone of 30m from the face. OC Props are fixed over wooden planks wherever height is more. 11. Cement injection was done along dip side.
    • 9.3. Method of Girder Support in the high roof. a. Span of 4/8 girder was exactly 1.5m b. Thorough dressing done before locating the girder notches. c. 3’ Notches were placed along dip side and 1.5’ notches along rise side. d. 0.5m of girder length were be seated inside the notches on either side. e. 3/69 laggings spreaded at equal space and the dip side member was kept very close to the side. f. Only normal sleepers shall be used for spreading over 3/6 girders. g. One cog with special sleepers (1.5m length x 5” width x 5”height) at center of the gallery, r ma constructed and jammed to stone roof by placing over 2 adjacent 4/8 girder. 9.4 Longwall face retreat below high roof Ku 1. 18 Nos girders were fixed at the height of 1.5 to 1.6M from marker band. Last 6 girders were fixed with slightly raising trend so as to have clearance below the girders for men and sh material passage. 2. All the girders were fixed in coal notches only. ure 3. Normally the LW face is worked in the middle section 3.0M having 1.0M above and 2.0M below a reference band which is known as ‘Marker band’. The Main gate and tailgate have D.S been developed with the similar section only. The available section over the marker band is 0.3M shale and 1.4M coal. Out of 1.7M, 1.0M is being extracted leaving 0.7M coal in the r:M roof as contact roof over the canopies of power supports. 4. But to optimize the floor regarding the section will be slightly lifted in the tailgate portion by 1.5M/1.5M with respect to marker band. tho 5. Thus0.5M to 1.0M floor removing is required from 315to 335M to expose 1.5M floor below marker. Au 6. Floor regarding in this Zone will be attempted with out stopping the face P/Shift regarding. 7. 1.5M below the marker band will be exposed by floor regarding. 8. Max depth to be regarded is 2.0M (335M to 375M). 9. Regrading will be done in a phased manner. Face operation will be stopped and the Face crew will be utilized for the purpose. 10. It will be also ensured that the cement injection and bamboo bolting to dip side is Completed before heightening.
    • 9.5 The floor Heightening:a. All vertical supports will be removed in the zone where floor to be removed.b. Controlled floor blasting.c. Fixing 4/8 girders at 1.5M apart with 3/6 lagging, wire mesh and wooden cogs with special timber (5”x5”x1.5M) to stone roof.d. Girder notches will be located in the coal sides only which will be grouted immediately.e. These girders will be re in forced with 3/6 girder ties.f. Removing the muck and filling on to AFC.g. Bolting the newly expose sides r ma Steel bolt –Rise side Bamboo bolts – Dip sideh. The removed vertical supports will be reinstalled below the girders.i. Wooden cogs will be erected below each 4/8 girder only on rise side to reduce the width of Ku the gallery exposure.j. Retreating the face. sh ure D.S r:M tho3/6 Bearings 4/8 Girders Linked fence 1.5M AuWherever the span was more than 3.0m two cogs were constructed over the girder (Ref. Fig: 2) 0.5 Width 4.0M 0.5
    • REDUCING THE FACE LENGTH from 166m to 150mPANEL NO 8 150m 150*420m 166mcutting 1:3 ratio mg to tgAt mg prog of 89.0 m and tg prog 127.40 m face length was reduced to 150m r ma Ku sh ure D.S r:M thoAu
    • arStone intrusion in the Seam um Seam 9m hK 3m 3m res 3m Su D. C-85 C-75 r:M tho Au
    • CONCLUSIONS Though the mine was not exclusively planned for Longwall technology, Padamavathi Khani Longwall project has been introduced and completed its 10 years service. 12 Longwall panels have been completed by overcoming different Mining, equipments, Quality, Geological problems. 10 years of LW Operations in the mine, brought out number of experienced officers ,supervisors and team of technicians and work men. Panels have been formed and extracted right from the out crop and upto r 300 m depth in panel No.8 with different walking distances. ma Experiences gained in the deviation panel (P-5C), Shallow depth panel (P-1 & 1A) and stone roof panels (P-21) were markable one. Ku Strata monitoring studies are being conducted by Mine Management sincerely and the health monitoring of supports are being done and maintained efficiently. With that, all these panels sh have been worked without any major strata control problems. This is the only project having worked all panels with 150 m face length (except P No 1 & ure 1A) and planned to continue to work with the same. Longwall production has crossed 1.1 M Te in the year of 1996-97. D.S Longwall production slowly reduced to 0.3 to 0.4 M Te in the last five years due to ageing of equipments and indegenisation. r:M At this end, new strategy is under formulation to reconstruct the PVK Longwall by refurbishing the equipment and by taking up necessary repairs and modifications for gainful development to extract the further LW panels so as to make the project economically viable. tho AuAcknowledgements:The authors expressed their gratitude to the management S.C.Co.Ltd.,for giving permission topublish the paper. The views expressed in this paper are of their own and not belonging to theorganisation in which they are working.
    • References:1. Report on “Numerical modeling & Strata and support behavior investigations at panel no.21 PVK-5 Incline”, Dec 04.2. Sarkar SK (1998 ) “ Mechanized Longwall Mining –The Indian Experiences ”3. Dr.Samir Kumar Das (2004), “ Design of Powered supports for Longwall faces” In house short term course for Mining Executives,4. Venkata Ramaiah M.S and Suresh Kumar M.D.,(2004) “Experience of Strata monitoring studies in shallow depth longwall extraction by caving in panel no.1A & 1 of PVK –5 Incline” r 3rd National seminar on rock excavation techniques at Nagapur organised by The Indian ma Mining and Engineering Journal Bhubaneswar chapter.5. Suresh Kumar M.D.,and U.Shiva shinkar (2006) “Need for working Longwall under hard Ku roof in future under ground mining–An experience of negotiating main weighting in sandstone roof “-Workshop on future of underground coal mining in India Mechanised board&pillar or sh longwall”organised by JMMF.Kolkata6. “Compendium on experiences of Longwall Mining Technology"by SCCL. ure7. Venkata Ramaiah and Suresh Kumar M.D.,( 2005 ) “ An experience of loading pattern with Sandstone roof in Longwall panel No.21 of Padavathi Khani No.5 Incline – National D.S Storming session on Mechanisation of Underground Coal Mines Challenges and Technical options. r:M tho Au
    • Au tho r:M D.S ure sh Ku ma r